israel sealed off the main roads around gaza. will israel invade on the ground? >> i think the chances are going up. >> jeffrey white, a former analyst with the defense intelligence agency says an israeli ground invasion of gaza would be a brutal bloody grind. >> there's a pretty high density of population throughout the strip. it's highest in the major areas, gaza city, there are a lot of civilians in other places as well. but the other part of this is that hamas fights from inside the cities. >> cities of narrow streets, bazaars, apartment buildings, translation? a punishing building-to-building slog in a place that's slightly more than twice the size of washington, d.c. we used a google map with cnn contributor general james marks. >> what kind of close combat are we talking about here? >> clearly what we have here in gaza city there are about 500,000 people that live in this city. and you can only imagine the type of combat that's got to take place in this very restricted terrain. >> terrain where marks says israeli troops will be exposed to ambush, sniper fi